1. Field of the Invention
The invention concerns a protective device, in particular for work cabs in a polluted atmosphere.
The invention is more particularly concerned with the vital protection of personnel against solid, liquid and gaseous products used in agriculture, especially soil and crop treatment products.
2. Description of the Prior Art
The state of the art is defined by certain types of apparatus which are more often than not fitted to the roof of a vehicle cab, the roof of an agricultural tractor cab, for example.
At present, apparatus of this kind consists of an enclosure having an air inlet externally of the cab leading to a motor-driven fan providing a pulsed air feed into the enclosure in which the air is passed through a dust filter followed, in certain installations, by an activated charcoal filter to absorb noxious gases, following which the filtered air is fed into the cab of the vehicle.
In this type of apparatus the motor-driven fan is mounted on one of the external walls of the enclosure and draws in air from the outside via an opening whose inlet surface area is substantially the same as the active working area of the blades of the fan, which gives rise to problems if any attempt is made to pressurize the cab fitted with this type of apparatus. It has been established that an air flowrate of approximately 900 m.sup.3 /h was essential if a particular tractor cab was to be properly pressurized, whereas the air pressure after filtration was no more than approximately 210 m.sup.3 /h, by virtue of head losses. If a flowrate of this magnitude is not achieved, a certain quantity of dust and polluting agents remains in the cab and the safety of the occupant is not assured.
Another problem with this type of known apparatus is the rapid clogging of the filter elements, and above all their rapid neutralization when the vehicle works in an atmosphere laden with liquid particles (as in spraying aqueous solutions), in which case the surrounding mist resulting from such spraying is drawn in by the motor-driven fan and directed pulse fashion onto the mechanical filter through which it passes, still laden with moisture and toxic products, to reach the chemical filter which very soon loses its adsorbent capacity, once it is impregnated with moisture.
A final known problem is the ease with which the airflow can circumvent the filters by passing through the interstices between the filters and surrounding structures.
A consequent objective of the present invention is to overcome these problems by providing an air pressurization and purification device which features a more efficient pressurization function such as to prevent any penetration of polluted air into the cab.
Another objective of the invention is to provide a device implementing more efficient purification and whose effective service life is enhanced on the one hand by delaying clogging of the filter elements and on the other hand by eliminating, after filtering, the majority of the liquid particles which may be contained in the outside atmosphere, and finally by forcing the air drawn in from outside through filters and a cell.
A further objective is to provide a device of this kind with combined monitoring means providing a continuous indication of the effectiveness of the pressurizer/air conditioner fitted to the working or driving cab. The pressurizing devices are equipped with an inlet filter, at least one physical filter, in some instances a chemical filter and a variable flowrate turbofan which provides a pulsed air feed to the operating cab.
It has been found that the efficiency of filtration in devices of this kind is dependent on numerous factors, two of which are essential. These two factors are, on the one hand, the relative pressure difference between the environments inside and outside the cab, and, on the other hand, the condition of the filters (inlet filter, physical filter and chemical filter). There is a high concentration of noxious products (sprays, powders) in the outside environment, and the inside environment is that which is to be protected. This pressure difference is achieved by pressurizing the cab using the motor-driven fan in the device. The pressure difference may be low, of the order of a few millimeters of head of water. This slightly increased pressure constitutes a barrier to the entry of noxious products into the cab. The efficiency of filtration is affected by three phenomena:
Clogging of the physical filters: the principal function of the inlet filter and the physical filter is to physically retain particles in suspension in the air. These particles are the primary support for noxious products and vapors. It is therefore understandable that in fulfilling their function these filters will become charged with particles and will progressively become clogged. Clogging results in another effect, the phenomenon of desorption; the filters, overloaded with particles, begin to release them and these particles reach the interior of the cab. The efficiency of filtration is then zero. PA1 Saturation of the chemical filter: the chemical filter, consisting of activated charcoal and a support, has the property of retaining the organic constituents of noxious aerosols and vapors by virtue of the phenomenon of absorption. Nevertheless, the retention capacity (in terms of molecular weight) of these chemical agents is limited and proportional to the weight of activated charcoal. The (non-limiting) retention capacity of the current cell is between 260 and 350 grams. Beyond this figure, its capacity falls rapidly and there is virtually no chemical filtration. Also, under these conditions, the cell is subject to the phenomenon of desorption.
Thus in the cases of devices of the kind in question, it is important to know at all times the condition of the chemical filter, the state of the mechanical filter(s) and the air flowrate, which must be adjusted to the minimum compatible with maintaining pressurization in the cab to economize on the filters, according to the filtration capacity of the filters.
The kind of protective device for working in a polluted atmosphere with which the invention is concerned is particularly designed for the driving cabs of agricultural tractors and comprises an air treatment unit mounted on or incorporated into the roof of the cab, equipped internally with at least one coarse filter unit for eliminating solids such as leaves and grit, a composite fine filter element for removing dust and formed by a stack of several materials of different nature facilitating the capture of dust particles, and an activated charcoal cell for removing noxious gases and vapors by adsorption and, finally, a turbofan for drawing in air from the outside and feeding it, filtered and purified, into the cab. Leaks on the air path are eliminated and filtration is rendered highly efficient by the arrangement proposed by the present patent. The filter and the cell are pressed onto an intermediate metal frame, the filter mounted on the inlet side being pressed naturally onto this frame by the reduced pressure, the cell being retained firmly by means of stays, for example, preventing its removal by virtue of the reduced pressure, the filter and the cell comprising a seal on the active surface. Given these conditions, it is merely necessary to provide a seal around the intermediate metal frame.
A device of this kind is disclosed in United Kingdom Pat. No. 82 29027. According to this patent, there are provided, in addition to pressurization monitoring means, toxicity detectors consisting of several pellets of reagent having the facility for changing color above a certain threshold of presence of pollutant chemical substances in the cab. Each pellet corresponds to a particular product routinely used in agriculture. Unfortunately, the reaction of the pellets to pollution is relatively slow, as a result of which these detectors are not able to indicate quickly a sudden rise in the toxicity concentration.
A first object of the present invention is a protective device equipped with electronic detection means which respond instantaneously to a change in the internal atmosphere of the cab. Moreover, the detectors are regenerated automatically, unlike the pellets, eliminating an ancillary task.
The device according to the aforementioned patent, although of good reliability, has nevertheless met with a certain amount of reluctance among users little aware of the dangers to which they are exposed when spreading products dangerous to their health. Apart from sometimes severe feelings of claustrophobia, some users complain of excessive cold or heat when atmospheric conditions are severe, in particular due to the fact that tractor cabs feature large glazed areas.
A second object of the present invention is to provide effective air conditioning irrespective of atmospheric conditions so that the driver may remain enclosed without suffering as a result of the inclemencies of the weather.
Also, in the aforementioned patent, the air is drawn in from outside and treated continuously. Pressurization is indispensible only when spreading dangerous products. The present invention provides for defining two sets of operating conditions of the protective device. The so-called "comfortable" operating conditions apply when carrying out transportation or other tasks which are not dangerous, and involve recycling the air in the cab, sparing the filters. The so-called "safe" operating conditions involve drawing in air from the outside and treating it, creating an increased pressure inside the cab. This pressurization is not obtained under the "comfortable" operating conditions since the cab is not sealed.
Furthermore, with a view to further enhancing the safety of the driver, the invention provides for distributing the filtered air by means of a special diffuser so that it forms an air curtain isolating the driver from the total volume of the cab under dynamic conditions.